Retroreflective conspicuity articles (e.g., microsphere-based and cube corner or prismatic type retroreflective articles) have been developed for use to increase safety and conspicuity especially during periods of reduced visibility. A wide variety of arrays of cube corners with various geometries have been disclosed. It is known to cover the cube corner elements with a sealing layer to maintain effective retroreflective performance. See, for example, U.S. Pat. No. 4,025,159 (McGrath).
It is desirable that a retroreflective article adhere to a desired substrate for the life of the substrate, or until intentional removal is desired. Difficulties have been encountered in attaching retroreflective sheetings to flexible polymeric substrates, such as highly monomericly plasticized polyvinyl chloride ("PVC") coated fabrics, without interfering with the life and function of the substrates. Articles which use polymer-coated or polymer-sealed fabric materials, such as a trailer tarpaulins and some roll-up signs, typically have a life span of about three to five years and up to about ten years. Roll-up signs are frequently used by road construction crews to designate work zones, road hazards, and the like. Polymer-coated fabric vehicle covers are particularly convenient, permitting the operator of the vehicle to gain access to the inside of trailers quickly and conveniently, and to maintain reasonable weatherproofing. The vehicle operator may open and close the fabric cover many times each day. Therefore the cover should be flexible but strong. Polymer-coated and polymer-sealed fabrics preferably withstand harsh weather conditions as well as the mechanical demands placed on them by the vehicle operator (in the case of trailer tarpaulins) and construction workers (in the case of roll-up signs). Trailer covers and roll-up signs may encounter extremes in temperature, chemical challenges from atmospheric pollution and road salt, and photo-reaction involving infrared, visible, and ultraviolet radiation from sunlight. A retroreflective sheeting attached to such a device preferably remains flexible and weatherproof throughout the expected life span of the article.
Many polymer-coated and polymer-sealed materials comprise a woven fabric layer of polyester, nylon, or cotton, coated or sealed on one or both major surfaces with a polymer preferably suited for the desired use. A commonly used polymer is highly monomericly plasticized PVC. Highly monomericly plasticized PVC is durable and convenient to handle because it is normally melt-bondable to itself or some other compatible polymer with the use of heat or radio frequency (RF) welding. Large fabric materials coated with PVC are manufactured by welding smaller panels together. Torn or damaged PVC coated fabric materials are often repaired while still on the vehicle.
Problems arise when attempting to form a sustainable retroreflective article (as measured by T-peel testing or other similar methods) by melt-bonding a retroreflective sheeting made of polymeric materials which are incompatible (from the standpoint of being incapable of forming a strong and durable melt-bond) with materials typically used for trailer tarpaulins or roll-up signs, such as monomericly plasticized PVC and copolymers of ethylene and comonomers (such as acrylic acid or vinyl acetate). An example of a pair of materials exhibiting melt-bonding compatibility is highly plasticized PVC and polyurethane. An example of a pair of materials having melt-bonding incompatibility is highly plasticized PVC and polycarbonate (a material frequently used in retroreflective sheetings) because of the substantially higher melting temperature of polycarbonate. Another example of a pair of materials having melt-bonding incompatibility is highly plasticized PVC and crosslinked acrylic cube-corners. Monomeric plasticizers present in the tarpaulin typically weaken the melt-bond and cause loss of its cohesive strength.
RF welding accomplishes fusion of polymeric materials through the presence of polar groups converting the radio frequency ("RF") energy into kinetic motion which heats the polymer. When a radio frequency field is applied to a thermoplastic polymer which has pendant polar groups, the ability of the polar groups to switch orientation in phase with the radio frequency determines the degree to which RF energy is absorbed and converted to kinetic motion of the polar group. This kinetic energy is conducted as heat to the polymer molecule; if enough RF energy is applied, the polymer will heat sufficiently to melt. A useful measure in determining the degree to which a polymer will absorb energy from an alternating field is the relation of the polymer's dielectric constant and the dielectric dissipation factor known as the loss factor and is given by the following relationship: EQU N=5.55.times.10.sup.-13 (f)(.Fourier..sup.2) (K) (tan .delta.)(1)
where N is the electric loss in watts/centimeter.sup.3 -second ("watts/cm.sup.3 -sec"), f is frequency in Hertz/sec, .Fourier. or Tau is field strength in volts/cm, K is the dielectric constant, .delta. or Gamma is the loss angle, and tan .delta. is the dissipation factor.
The dissipation factor is the ratio of the in-phase to out-of-phase power. If the polar groups in a thermoplastic polymer have a relative inability to switch orientations in the RF field, this results in a phase lag known as the dissipation factor. The higher the dissipation factor, the greater the amount of heat a RF field will generate. Studies with thermoplastic polymers and radio frequency welding have demonstrated that thermoplastic polymers with dissipation factors of approximately 0.065 or higher can form useful welds. For example, PVC has a dissipation factor of approximately 0.09 to 0.10 at 1 MHz, nylon caprolactam has a dissipation factor of 0.06 to 0.09 and polycarbonate has a dissipation factor of only 0.01 at the same frequency. The dielectric constants for these three compounds are 3.5, 6.4, and 2.96, respectively, at 1 MHz.
Polyethylene, polystyrene, and polycarbonate have very low dissipation factors and in practical use have poor radio frequency welding capability. Polyvinyl chlorides, polyurethanes, polyamides, and polyesters have reasonably high dissipation factors and have been found in practical use to form very functional RF welds. Reference is made to the article "RF Welding of PVC and Other Thermoplastic Compounds" by J. Leighton, T. Brantley, and E. Szabo in ANTEC 1992, pps. 724-728. These authors did not attempt to weld polycarbonate to the other polymers because of the understanding in the art that a useful weld, using RF energy, would always fail to form.
Only those polar groups within the RF field will be put into motion and thus subject to heating. In addition, RF fields can be applied in well defined zones or fields as compared to thermal heat application methods. As a result RF welding techniques can be used to readily achieve melt-bonds in desired locations with little need for thermal insulation.
PCT Application No. WO 93/10985 (Oppenhejm), published Jun. 10, 1993, discloses, inter alia, attaching PVC retroreflective articles to a tarpaulin cloth coated with PVC using RF welding. This composite article can then be hot air fused to a tarpaulin vehicle cover also coated with PVC. To thermally weld the PVC coated cloth to the PVC coated tarpaulin cover, the two surfaces are heated with air at approximately 400.degree. C. to 600.degree. C. and the surfaces then pressed together to accomplish the hot air fusion. The purpose of the intermediate tarpaulin cloth attachment is to provide thermal insulation between the hot air and the retroreflective article attached to the tarpaulin cloth to prevent thermal melting, loss of retroreflection, and destruction of the retroreflective article.
A commercially available product from Reflexite Corporation, believed to have the designation 393-2457-372, comprises cube corner retroreflective sheeting having PVC cube corners, the cube corners melt-bonded to a PVC-coated fabric. Cube corner retroreflective articles having the cube-corners constructed from PVC have relatively low coefficients of retroreflectivity, generally on the order of 250 candelas/lux/square meter or less for clear, colorless sheeting.
Assignee's U.S. application Ser. No. 08/236,339, filed May 2, 1994 abandoned, and Ser. No. 08/434,347, filed May 2, 1995, pending describe a high brightness, flexible, durable, retroreflective article comprising a polymeric cube corner type retroreflective layer having a polymeric compatibilizing layer for attachment to a flexible polymer-coated fabric material. The retroreflective sheeting comprises, in order: a polymeric cube corner retroreflective layer, a polymeric compatibilizing layer, and a flexible polymeric coated fabric, wherein the coated fabric has a non-compatible polymeric coating on each major surface. In these constructions there is thus always at least one non-compatible polymer layer between the fabric and the retroreflective layer. The compatibilizing layer is a polymeric material having characteristics suitable for bonding to both the retroreflective layer and a polymer-coated or polymer-sealed fabric material under conditions using high frequency welding and/or patterned thermal welding. All methods of making these retroreflective articles involve providing a separate polymer-coated fabric having the fabric portion completely engulfed on both major surfaces with a non-compatible polymer, and thereafter heat or RF welding all three layers together. The use of a separate polymeric compatibilizing layer and a non-compatible coated fabric layer increases cost as well as the thickness of such articles, therefore tending to reduce their flexibility and yielding articles having higher bending radii. It would be highly desirous if retroreflective articles could be produced which do not degrade in retroreflectivity or adherence of the retroreflective layer to the substrate with time, remain flexible by virtue of decreased thickness, and preferably are less expensive and easier to produce.